Field of the invention

ABSTRACT

Device for connecting, by means of a shielded cable, electrodes to a measuring device, located on either side of a wall ( 18 ) separating an enclosure under pressure from the outside environment.  
     The device comprises a rigid protector sleeve ( 16 ) made of an insulating material that tightly runs through the wall and extends to the immediate vicinity of the electrode, into which shielded cable (C) is passed. A tube ( 23 ) made of a conducting material is arranged in rigid sleeve ( 16 ) and in electric contact with the cable shield. A plug ( 24 ) is secured to rigid sleeve ( 16 ). It is connected to electrode ( 15 ) and electrically linked to core ( 21 ) of the shielded cable inside metal tube ( 23 ). An electric connector ( 20 ) is associated with rigid sleeve ( 16 ) outside wall ( 18 ) for connection of a shielded wire connected to the measuring device. This device can be used in a system measuring the electrical resistivity of a sample in a frequency range that can reach several ten MHz.  
     Applications: petrophysical measurement on porous rock samples for example.

FIELD OF THE INVENTION

[0001] The object of the present invention is a device for precisely establishing in the laboratory the curve of the resistivity index of a solid sample independently of the capillary pressure curve, suited for high-frequency measurement.

[0002] Measurement of the resistivity index of small cores is necessary to obtain a precise estimation of the water saturation from log data obtained for example by means of the measurement while drilling (MWD) technique.

BACKGROUND OF THE INVENTION

[0003] Patents FR-2,781,573 and FR-2,762,681 (U.S. Pat. No. 5,979,223) filed by the applicant notably describe methods and devices intended for continuous measurement of the resistivity index curve of a solid sample initially saturated by a first wetting fluid, such as a geologic sample, independently of the capillary pressure curve. The porous solid sample is contained in a sealed sheath placed in an elongate containment cell between two terminal parts. Channels provided through the two terminal parts communicate with an injection system allowing to inject a second, non-wetting fluid into the sample at a first end of the cell and to drain the first fluid from the cell at the opposite end, through a semipermeable membrane permeable to the first fluid. The sample is contained in a sheath and subjected to a radial pressure by injection of oil under pressure into the annular space between the body of the cell and the sheath. A membrane wettable only by the second fluid is interposed between the sample and the first end of the cell for re-imbibition operations.

[0004] Electrodes interposed between the sample and its sheath allow to apply an electric current and to detect potential differences that appear between distinct points in response to the application of an electric current. The electrodes are connected to a device measuring the complex impedance of the sample. The longitudinal extension of the electrodes is relatively great in relation to the length of the sample so that the largest possible part of the volume of the sample is involved in the impedance measurement while avoiding short-circuits through the ends of the sample likely to distort the measurements.

[0005] One or more injection pressure stages are applied, the continuous variations of the resistivity index as a function of the mean saturation variation are measured without waiting for the capillary equilibria to be established.

[0006] The annular space between the sheath and the external wall of the cell being under high pressure, the electric conductors connecting the electrodes to the measuring device run through the external wall of the cell through sealed bushings (glass bead connectors for example).

[0007] Studies show that the resistivity index of porous rocks varies substantially with the frequency. As logging sondes measure the electric resistance of the formations crossed at often very high frequencies, they must be able to work with precision in the same frequency range in order to allow to really compare the measurements obtained by means of the well tools to the resistivity index measurements obtained in the laboratory by means of the cells.

[0008] The results obtained with the previous cells are satisfactory when the frequency range of the electric currents applied remains within the limit of some KHz or several ten KHz. They lose a lot of significance when the impedance measurements are carried out at much higher frequencies ranging for example between 1 MHz and 10 MHz. At such frequencies, shielded cables of constant impedance must of course be used. Continuous connection of the electrodes to the measuring device by means of shielded cables is difficult to achieve because of sealing problems. If a conventional connector of glass bead type is used for example, this leads to a break in the continuity of the shielding. This discontinuity, which would have no notable effect at low frequencies, is the source of parasitic reflections and of a significant attenuation of the signals at high frequencies.

SUMMARY OF THE INVENTION

[0009] The device according to the invention allows to connect, by means of a shielded cable, at least one electrode to a measuring device, located on either side of a wall separating an enclosure under pressure from the outside environment. It comprises at least one rigid protector sleeve made of an insulating material that tightly runs through the wall and extends to the immediate vicinity of the electrode, wherein the shielded cable is passed, this rigid sleeve containing a tube made of a conducting material that is in electric contact with the shield of the cable, and being rigidly and tightly associated with a connection means connecting electrically the core of the shielded cable to the electrode.

[0010] The electric connection means comprises for example a plug connected to the electrode, with a baseplate that is rigidly and tightly fastened in a cavity of the sleeve, and electrically connected to the core of the shielded cable.

[0011] In order to take into account a certain possible freedom of motion of the electrode, the plug is engaged in a hollow of the electrode and suited to maintain the electric contact with the electrode in which it moves.

[0012] According to an embodiment, the device comprises an electric connector associated with the rigid sleeve outside the wall for connection of a shielded wire connected to the measuring device, a shielded cable element within the rigid sleeve whose core is connected to the connection means, and the shield is electrically connected to the conducting tube that extends to the inside of the rigid sleeve as far as the zone where the core is connected to the connection means.

[0013] The wall is for example the wall of the body of a cell intended for measurement of the variations of the resistivity index of a porous solid sample embedded in a sheath and subjected to a radial pressure by injection of a liquid under pressure into the body of the cell, these variations being the result of operations of forced displacement of a first fluid out of the sample by injection of a second fluid, one of the two fluids being electricity conducting, unlike the second one, by means of electrodes arranged between the sample and the sheath and provided each with an extension running through the sheath, each rigid sleeve running through the wall of the cell body and extending substantially to the sheath.

[0014] The measuring system according to the invention comprises an elongate containment cell for a sample in a sheath, means for injecting a liquid under pressure into the body of the cell so as to exert a radial pressure on the sample, electrodes arranged between the sample and the sheath, allowing application of an electric current and detection of the potential differences that appear between distinct points of the sample in response to the application of the electric current. The electrodes are each provided with an extension running through the sheath and connected to a device measuring the impedance of the sample, outside the cell body, a first semipermeable filter permeable to the first fluid and arranged substantially in contact with a first end of the sample, and injection means for injection under pressure of a second fluid through a second end of the sample. The system comprises connection devices for connecting the various electrodes to the measuring device by means of shielded cables and each connection device comprises at least one rigid protector sleeve made of an insulating material that tightly runs through the wall and extends to the immediate vicinity of the electrode, wherein the shielded cable is passed, this rigid sleeve containing a tube made of a conducting material in electric contact with the shield of the cable and being rigidly and tightly associated with a connection means connecting electrically the core of the shielded cable to the electrode.

[0015] The electrodes preferably have a relatively great longitudinal extension in relation to the length of the sample (between ¼ and ¾ of the length of the sample and preferably of the order of ½) but smaller than this length, so that the largest possible part of the volume of the sample is involved in the impedance measurements while avoiding short-circuits through the ends of the sample.

[0016] The connection device defined above is advantageous in that it allows a shielded wire to run tightly through a wall without any discontinuity of the core and of the shield of the cable likely to affect the signals transmitted, in a frequency range that can reach several ten MHz.

[0017] The measuring system with its connection device(s) as defined above is particularly advantageous in that it allows:

[0018] to establish a very precise curve of the continuous resistivity index during drainage and in a short time (about 2 days for a typical 100 mD sandstone whereas the typical time required using the continuous injection technique is often of the order of two weeks);

[0019] the incidence of non-uniform saturation profiles during measurement is negligible. This is due to the combination of three factors: (i) the radial resistivity measuring technique, (ii) the presence of semipermeable filters at the outlet, (iii) the total volume of the core is analysed by means of electric measurements (which is verified when the diameter of the core is greater than its length);

[0020] to provide precise resistivity index measurements in a very wide frequency range up to frequencies of the order of several ten MHz.

BRIEF DESCRIPTION OF THE FIGURES

[0021] Other features and advantages of the invention will be clear from reading the description hereafter of a non limitative embodiment example, with reference to the accompanying drawings wherein:

[0022]FIG. 1 diagrammatically shows, in longitudinal section, a measurement cell allowing to measure the resistivity of a porous sample,

[0023]FIG. 2 is a cross-sectional view of the arrangement of the electrodes around a sample allowing to inject an electric current and to measure the potential difference generated by the current getting through the sample,

[0024]FIG. 3 diagrammatically shows, in longitudinal section, a connection device allowing sealed electric connection of a shielded cable connecting an electrode to an external measuring device,

[0025]FIG. 4A shows the compared variations of the impedance modulus Z of an electric test circuit placed outside the cell (solid line) and inside the cell and connected to the impedance meter by means of the connection device described (dotted line),

[0026]FIG. 4B shows, under the same conditions, the Argand diagrams (real part of Z in abscissa and imaginary part of Z in ordinate) that correspond thereto,

[0027]FIGS. 5A, 5B respectively show the compared variations of the normalized impedance modulus as a function of the frequency for two Fontainebleau sandstone samples that are respectively water wet (ww) and oil wet (ow) and for saturations Sw of 1 and 0.38 respectively,

[0028]FIGS. 6A, 6B show, under the same conditions, the Argand diagrams that correspond thereto respectively, and

[0029]FIGS. 7A, 7B show the dispersive effects of the frequency on the curves representative of the variation of the resistivity index as a function of the brine saturation, for two Fontainebleau sandstone samples, one being water wet (ww) (FIG. 7A), the other oil wet (ow) (FIG. 7B), and the fast reduction of the slope of the curves above 500 KHz.

DETAILED DESCRIPTION

[0030] The connection device is described hereafter in a non limitative way in connection with an experimental system intended for measurement of the resistivity index variations of a porous solid sample, due to forced displacements of a first, electricity-conducting wetting fluid such as brine for example, by injection of a second, non-conducting fluid such as oil for example (drainage stage), or of the second fluid by the first (imbibition stage) as described in the aforementioned patents filed by the applicant.

[0031] It comprises (FIG. 1) a containment cell intended for a core, comprising a hollow body 1 of cylindrical symmetry closed at its two opposite ends by two terminal parts 2, 3. Sample S is placed in a cylindrical elastomer part 4 whose U-shaped longitudinal section forms a sheath for sample S. All of sample S and of sheath 4 is placed in an inner cavity of body 1 and is axially delimited on either side by the two terminal parts 2, 3. On the side of terminal part 2, sample S is in contact with a semipermeable filter 5 wettable by the first fluid, such as a ceramic filter. On the side of opposite terminal part 3, sample S is in contact with a membrane 6 wettable by the second fluid. The inner faces of terminal parts 2, 3 are provided with a network of grooves 7 (FIG. 2). Fastening means (not shown) allow the two terminal parts to be rigidly fastened to each other.

[0032] Channels 8 run through terminal part 3 and communicate the network of grooves 7, on its terminal face, with a first source 9 delivering the second fluid under pressure. Similarly, channels 10 run through terminal part 2 and communicate the corresponding network of grooves 7 with a second source of pressure 11 of the first fluid drained out of the sample as a result of the injection of the second fluid. An element 12 is installed on circuit 10 to measure the volume of fluid driven out of sample S. A low-cost capacitive pickup having a 0.05 cc precision and a 0.01 cc resolution, similar to the pickup used in the device described in patent application FR-2,772,477 filed by the applicant, is preferably used.

[0033] The device comprises for example two pairs of electrodes E1, E2 moulded in sheath 4 so as to be closely pressed against the peripheral wall of the sample, allowing application of an electric current. The potential difference V created in response to the application of the electric current is measured by means of another pair of electrodes E′1, E′2, likewise moulded.

[0034] This separate allocation of the electrode pairs, one to application of an electric current, the other to potential differences measurement, allows to avoid resistances due to contacts. The electrodes are for example square in shape and made of Monel. The angular extension of a pair of electrodes around the sample is less than 90°. Their length must be shorter than the length of the sample so as to avoid end short-circuits outside the sample, directly through the fluids, which would distort the measurements. However, their length must be great enough in relation to the length of the sample so that the current lines cover the most part of its volume with a relatively even distribution. This length can vary considerably according to the diameter of the sample. In the experiments carried out, it has been found that the length of the electrodes can advantageously range between ¼ and ¾ of the length of the sample, and it preferably is of the order of half of this length.

[0035] Annular space 13 between body 1 and sheath 4 communicates with pressure means 14 allowing injection of a liquid under pressure that exerts a radial confining pressure on sample S. The radial confining pressure around the sample is for example of the order of some MPa, sufficient to ensure good electric contact of the electrodes. Thus, under normal conditions, the contact resistance is generally of the same order of magnitude as the resistance of the sample that has to be measured with a low water saturation.

[0036] The assembly is placed in a thermostat-controlled enclosure (not shown).

[0037] All the electrodes E are provided with a hollow extension 15 running through the thickness of sheath 14, and they are connected to an RLC impedance meter 16 coupled with a measurement acquisition device 17 by the connection device described hereafter.

[0038] The connection device comprises for each electrode E (FIG. 3) a tubular sleeve M made of an insulating material that fits into a bore N in the outer terminal wall 18 of body 1 and rigidly fastened thereto. Seals 19 are arranged in grooves of tubular sleeve M. A BNC type electric connector 20, well-known to the man skilled in the art, is for example fastened against the outer wall of tubular sleeve M. One of its ends is connected to the core 21 of a shielded cable portion, the other to the braid 22 of this cable. Braid 22 is in electric contact with a stainless steel tube 23 arranged in a cylindrical cavity of tubular sleeve M. The baseplate 24 of a plug fits into another cavity at the opposite end of this tubular sleeve M and it is fastened thereto by a threaded ring 26. Sealing is provided by seals 25. This baseplate 24 is provided with a first extension 27 on which core 21 of the shielded cable is welded and, at the opposite end thereof, it is secured to plug 28 intended to fit into a housing of extension 15 of each electrode E. In order to reinforce the electric contact with extension 15 of electrode E, plug 28 is provided with a leaf spring 29. A certain clearance is allowed for plug 28 in its housing in electrode E to take into account the displacements of elastomer sheath 4 when it is pressed against sample S through injection of liquid into annular space 13.

[0039] Stainless steel tube 13 extends to the inside of tubular sleeve M so as to cover and to electrically insulate the zone where the core of the cable is welded to extension 27. As it is connected to shield 22, core 21 is electrically insulated up to its junction with plug 24.

[0040] Wire 21 is relatively slack between connector 20 and terminal part 24 inside tubular sleeve M for assembly purposes.

Operation

[0041] Sample S saturated with the first fluid is placed in the enclosure and a radial confining pressure is applied by connection with pressure means 14.

[0042] A second fluid such as oil is then injected through channels 8 at a first pressure, and the variations of the complex impedance of the sample are continuously measured for several frequencies between 0.1 Hz and several ten MHz and recorded by acquisition device 16, 17. The data is analysed using a generalized resistivity index or impedance index according to the saturation and to the frequency f, defined as follows: ${{Ir}({Sw})} = {\frac{{Z({Sw})}}{{Z\left( {{Sw} = 1} \right)}} = {g\left( {{Sw},f} \right)}}$

[0043] where ${Z} = \left( {{{Re}(Z)}^{2} + {{Im}(Z)}^{2}} \right)^{\frac{1}{2}}$

[0044] It can be checked that the frequency has a strong effect on the resistivity index curves Ir above 500 KHz (FIG. 7A). For the water wet sample, the data can be adjusted by applying Archie's law, which is well-known, and the saturation index decreases by 2 to 1 KHz down to 1.5 to 2 MHz. For the oil wet sample, the influence of the frequency is different (FIG. 7B) and the curve is markedly non-linear in a log-log scale. At high frequencies, the difference in relation to the 1 KHz curve depends on the saturation. At 2 MHz, the difference appears with Sw=0.7, and the curve becomes gradually flatter with a low saturation. If a single point of the curve was measured at low saturation, a saturation index of 2 would be obtained.

[0045] As for the curves of FIGS. 5, 6, the dispersion curves can be extracted from the data recorded for two saturation values. For a 100% water saturation, only a minor difference can be observed at high frequency (FIG. 5A). The cutoff frequency (i.e. the vertex of the semi-circle in the Argand diagram) (FIG. 6A) is of the order of 5 MHz in both cases. At low frequency (between 0.1 and 1 KHz), the differences can be attributed to different surface roughnesses for the two samples. For a lower saturation (Sw=0.38), a cutoff frequency decrease can be observed (about 500 KHz). The phenomena attributed to the surface roughness have shifted to the lower frequencies (0.1 Hz).

Tests

[0046] In order to test the quality of the connection device, an electric circuit consisting of a resistor of about 1 KΩ and of a capacitor having a capacitance of the order of 200 pF, reproducing typically the electric behaviour of a sample S, is placed in the cell and connected to electrodes E, 15. The complex impedance Z of the circuit has been measured for all the frequencies up to 20 MHz when this circuit is placed outside and inside the cell, and connected by the connection device described above. It can be seen in FIGS. 4A, 4B that the results obtained are entirely identical and that the cell and the connection device do not in any way spoil the quality of the measurements. 

1. A device intended for connection, by means of a shielded cable, through wall (18) of an enclosure under pressure, of at least one electrode (E, 15) in the enclosure and of a measuring device outside the enclosure, characterized in that it comprises at least one tubular protector sleeve (M) made of an insulating material that is fastened to the wall and tightly runs right through it, this tubular sleeve (M) comprising an inner cavity into which shielded cable (C) is passed, a tube (23) made of a conducting material arranged in tubular sleeve (M) and in electric contact with shield (22) of the cable, an electric connection means (24, 28) comprising a plug (24, 28) connected to electrode (E, 15) with a baseplate (24) that fits into the inner cavity of tubular sleeve (M) and electrically connected to core (21) of the shielded cable inside conducting tube (23), and seal means (19, 25) associated with tubular sleeve (M) and with the electric connection means, to insulate the inside of the enclosure from the outside and the inner cavity of tubular sleeve (M).
 2. A device as claimed in claim 1 , characterized in that plug (24, 28) fits into a hollow of electrode (E, 15) and it is suited to maintain the electric contact with the electrode when the latter moves.
 3. A device as claimed in any one of the previous claims, characterized in that it comprises an electric connector (20) associated with tubular sleeve (M) outside wall (18) for connection of a shielded wire connected to the measuring device, a shielded cable element (21, 22) inside the tubular sleeve whose core (21) is connected to connection means (24, 28), and shield (22) is electrically connected to conducting tube (23) that extends to the inside of the tubular sleeve as far as the zone where core (21) is connected to connection means (24, 28).
 4. A device as claimed in any one of the previous claims, characterized in that wall (18) is the wall of the body of a cell (1) intended for measurement of the resistivity index variations of a porous solid sample (S) embedded in a sheath (4) and subjected to a radial pressure by injection of a liquid under pressure into the body of the cell, these variations being the result of operations of forced displacement of a first liquid out of the sample by injection of a second fluid, one of the fluids being electricty conducting, unlike the other, by means of electrodes (E) arranged between the sample and sheath (4) and provided each with an extension (15) running through the sheath, each tubular sleeve (16) running through the wall of the body of cell (1) and extending substantially to sheath (4).
 5. A system for measuring physical quantities of a porous solid sample (S) placed in a sheath (4), comprising an elongate containment cell for the sample in it sheath, means (14) for injecting a liquid under pressure into body (1) of the cell allowing to exert a radial pressure on the sample by injection of a liquid under pressure, and means intended for measurement of the resistivity index variations of sample (S) due to operations of forced displacement of a first fluid out of the sample by injection of a second fluid, one of the fluid being electricity conducting, unlike the other, comprising electrodes (E) arranged between the sample and sheath (4) and provided each with an extension (15) running through the sheath, and a measuring device (16, 17) outside the cell, connected to the electrodes, characterized in that it comprises at least one tubular protector sleeve (M) made of an insulating material that is fastened to the wall and tightly runs right through it, this tubular sleeve (M) comprising an inner cavity wherein shielded cable (C) is passed, a tube (23) made of a conducting material arranged in tubular sleeve (M) and in electric contact with shield (22) of the cable, an electric connection means (24, 28) comprising a plug (24, 27) connected to electrode (E, 15) with a baseplate (24) that fits into the inner cavity of tubular sleeve (M) and electrically connected to core (21) of the shielded cable inside conducting tube (23), and seal means (19, 25) associated with tubular sleeve (M) and with the electric connection means, so as to insulate the inside of the cell from the outside and the inner cavity of tubular sleeve (M).
 6. A system as claimed in claim 5 , characterized in that electrodes (E) allow to apply an electric current and to detect potential differences appearing between distinct points of sample (S) in response to the application of an electric current, and the cell comprises a first semipermeable filter (5) permeable to the first fluid and arranged substantially in contact with a first end of the sample, and injection means (8, 9) for injection under pressure of a second fluid through a second end of the sample.
 7. A system as claimed in claim 6 , characterized in that the electric connection means comprises a plug (24, 28) connected to electrode (E, 15) with a baseplate (24) that is rigidly and tightly fastened in a cavity of tubular sleeve (M), and electrically connected to core (21) of the shielded cable.
 8. A system as claimed in claim 7 , characterized in that plug (28) fits into a hollow of the electrode and it is suited to maintain the electric contact with the electrode when the latter moves.
 9. A system as claimed in any one of claims 5 to 8 , characterized in that it comprises an electric connector (20) associated with tubular sleeve (M) outside wall (18) for connection of a shielded wire connected to the measuring device, a shielded cable element (21, 22) inside the tubular sleeve whose core (21) is connected to connection means (24, 28), and shield (22) is electrically connected to conducting tube (23) that extends to the inside of the tubular sleeve up to the zone where core (21) is connected to connection means (24, 28).
 10. A system as claimed in any one of claims 5 to 9 , characterized in that it comprises electrodes having a relatively great longitudinal extension in relation to the length of the sample but smaller than this length, so that the largest possible part of the volume of the sample is involved in the impedance measurements while avoiding short-circuits through the ends of the sample. 